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Old 06-25-2017, 09:35 PM   #1
Mini-Skoolie
 
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Solar Hydronic Heating with Storage

So been sat around thinking today about how to power a school bus RV project, been running some numbers to try and make everything balance out. if this wild project does ever get off the ground i would like to explore pretty much the whole country hot cold and in-between. i also like the idea of not been tied to camp sites especially when out in the boon docks. So it was apparent pretty quickly that solar and batteries is quite economical especially the last few months the prices of panels seems to have been dropping i'm seeing 300w for $200. so 1200w installations are not out of this world expensive (any more). also 12v and 120v split AC options are totally doable. however try and store enough energy during the day in batteries to heat at night and the wheels fall off pretty quickly. the cost, volume and weight in batteries is unfeasible. however...

55g of water in a insulated tank with coils (heat exchanger) on a circuit that includes a fossil fuel heater, engine heater circuit, and the coach heater. this same circuit could also include the domestic hot water (with a backup heating coil of course).

on a circuit separate from the engine coolant / coach heaters would be a roof mounted solar panel that would heat the mass directly. from what i have read on the net a single panel (forgot the dims right now) would be plenty for a 40' bus.

something like this... black hose pipe...

https://www.amazon.com/dp/B00ULYKO9W...a-313605881933


you would probably want to isolate the engine when you were parked for any length of time to stop the system trying to keep it warm for no reason.

i also like this system because if plain water was used it could be dumped when not needed.

i mentioned this in another thread but its still border line not worth it with the cheap price of diesel and propane.

anyway i would be interested to hear constructive thoughts on the subject, positive or negative.
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Old 06-27-2017, 01:38 PM   #2
Mini-Skoolie
 
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OK i'm not letting this one go as i believe it is a good idea... see diagram attached... water has 4 times the specific heat capacity of air and air is 1200 times bigger than water per weight. so a bit of water can heat a bit of air for a long time. trying not to get too techie. i see this system all the time in domestic properties (google hydranic heating), just not mobile applications due to weight. however a skoolie was made to carry a lot of weight and in this application hardly ever moves.

i just want some attention
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Old 06-27-2017, 03:56 PM   #3
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Other than encouragement, I don't have much of anything to contribute. I am curious to see where the thread goes though.

I carry about 130 gallons of water most of the time. I see no reason why a few of those could not be circulating up to the roof, thru a collector, and back down to be used for something.

I've got solar photovoltaic and battery (lead-acid and lithium) experience and also did some playing around with a solar hot tub heater a number of years ago. There is no doubt that the sun is incredibly powerful and it can make water very hot with very little effort. Given this little experience with solar heated water, the trick seems to be keeping it all contained (not leaking). It seems much easier to do with electrons.
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Old 06-27-2017, 04:10 PM   #4
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I'm thinking of adding two solar water heating panels to complement my eight gridtie PV panels. They would be only for DHW, not for heating. The biggest potential issue I may encounter would be to stop the water inside them flashing into steam on a hot day, so some sort of thermostatically-controlled drainback system may be necessary, plus some pressure relief and over-temperature valves. I would like to be able to make them myself, if only because I can then make them the exact size needed. Like my PV panels now are, they would be tiltable to face the sun as well as possible on a bus roof. My thought is to use one long continuous coil of black pipe or tube per panel, so far fewer joints to potentially leak - whether I use a plastic hose or a painted copper or aluminum tube is one of several things I need to research. My intention is to heat all my DHW by solar during the summer, and to preheat the water at other times so I use less propane.

John
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Old 06-27-2017, 04:13 PM   #5
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We built a solar pool heater many years ago. We built a "ladder" looking mashup from 3/4 PVC pipe and T's. Spray painted it black and put it on the poolhouse roof.

No thermostat, we just diverted a small amount of water from the filter pump through it constantly.

After a week we had to modify it as the pool got too hot.....

No doubt that the energy is there.... Weight would be my biggest concern in a mobile installation.

I look forward to seeing your progress.
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Old 06-27-2017, 06:36 PM   #6
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I have built several solar water heating systems. Mainly for domestic water cause the sun isn't always there for heat in the winter time. I have installed many different designs.
I think the drain back system is the way to go for small systems to be able to control overheating and save power to the pump.
Another one is a storage/inertia tank that the loop maintains the temp. In and the whatever loops feed off of that but when temps are high then the storage loop would have to run continuous to help keep the temps down.
Ethylene glycol is the best for solar water heating because it can handle a lot higher and lower temps without boiling or freezing. A special designed flushing filling pump system is needed to get all of the air out because it is a closed loop system but I can tell you how to build it at home using the pump in/for your system.
A few main safety factors!
For any system you build.
You need a TMV thermostatic mixing valve that mixes cold water with whatever temp hot water to bring it down to a safe limit(120)adjustable before it goes to the fixture or lesser piping that can't handle high temp water.
There are several to choose from.
Some can go directly at the fixture but your piping to that point needs to be able to handle it.
Some can go directly at the tank/heat exchanger and handle the entire building/bus load. The mixing valves for direct tie in to the main to feed multiple thing have a safety built in to shut down all hot water if it can't control it for whatever reason. I can't really say the point of use (at the fixture ) ones do or don't.
Temperature relief valves piped outside to safe location(where no one normally walks,stands? Under the bus with a good splash radius?)
On the user side 120-safe 140 max and on the solar side 180-210 max or whoever it gets will look like a boiled lobster.
Most solar water tanks are a copper coil wrapped around a tank, insulated and skinned to look like a hot water heater. Regardless of your solar design? The quickest way to cool down a hot system is to cover the panel's/piping whatever with a reflective material (tent style) rescue blanket type idea to block the sun and provide an air space. Been there done that. Had a bad pump controller.
Good luck to all and I/my computer? Suck at pics but if you PM me I can e-mail/text directly.
Good luck
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Old 06-27-2017, 07:38 PM   #7
Mini-Skoolie
 
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Thanks all that was encouraging enough to start running some more numbers. As far as Skoolie operator goes I'm still in the 'dreamer' mode. I like PV as several have said but the problem of storing the energy from daytime till night becomes prohibitively everything,side, weight, cost. I was originally thinking 55 gallon but I'm wondering if I can make a smaller amount work with good insulation and dual zone underfloor heating I.e. Cabin and salon (saloon?).

I have done some reading and the thought of an off the shelf control system is appealing. Any recommendations ?

Also are the systems general pressurized?

Thanks all...
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Old 06-27-2017, 10:21 PM   #8
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I have been contemplating this and am beginning to wonder if solar water heating (in a manner practical on a bus) may work great during the summer but the simple uninsulated collectors that I have worked with are not worth a darn when the weather cools down enough that you need heat at night.

Cooler weather may dictate evacuated tube collectors and these may be a bit fragile for mobile installation.

I would certainly be looking a a diesel or propane fired backup for colder weather & shorter days.
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Old 06-28-2017, 08:25 AM   #9
Mini-Skoolie
 
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PNW maybe its time to wait and let the vacuum tube technology progress till its suitable for mobile installations...
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Old 07-05-2017, 05:41 AM   #10
Mini-Skoolie
 
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Solar Water Heater

Quote:
Originally Posted by Steven UK View Post
OK i'm not letting this one go as i believe it is a good idea... see diagram attached... water has 4 times the specific heat capacity of air and air is 1200 times bigger than water per weight. so a bit of water can heat a bit of air for a long time. trying not to get too techie. i see this system all the time in domestic properties (google hydranic heating), just not mobile applications due to weight. however a skoolie was made to carry a lot of weight and in this application hardly ever moves.

i just want some attention
I'm not sure how much hot water you need, but my friend put 2 x 100' sections of 1/2" black flexible pipe together on the roof of his bus, then plumbed it in. When in direct sunlight, you have water that you can barely keep in your hand in because it is at least 125 degrees. Yields approximately 3 gallons of water. I told him that he should try 3/4" pipe, to give you about 4 1/2 gallons.

He has been using it now for about three weeks or so and is very happy with it.

It cost him in the neighborhood of about $25.00 or so for all of the parts, and maybe two hours time coiling it. Copper would cost more, but I think it would hold the heat better. Still paint it black, but the copper would hold the heat.
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